The effect of multiple dosage on the kinetics of glucuronidation and sulphation of diflunisal in man

Dosing of antirheumatic drugs in renal disease and dialysis

Many patients with rheumatic diseases have their management complicated by renal problems. Renal failure modifies the metabolism of many drugs, especially by retention. Questions often arise about the effects of renal failure on the handling of drugs commonly used in rheumatology. For which drugs must we be especially concerned about increased toxicity? Patients on chronic dialysis may also need a variety of drugs for rheumatic disease. How are our drugs dialyzed, and which of these can be safety used and how best to use them?Decisions about dosing of rheumatic drugs are often required for the patients with chronic renal insufficiency or on long-term dialysis, although many drugs have not been formally studied in these settings. Patients with renal insufficiency are excluded from most drug trials. Data for some of these drugs have to be extrapolated based on the information available about the pharmacokinetics of the drug.This review addresses dosing of commonly used drugs in rheumatology in patients with chronic renal insufficiency or failure. It is compiled from a MEDLINE search of papers dealing with renal handling of antirheumatic drugs and suggestions for dose adjustments for these drugs. Drugs reviewed include commonly used disease-modifying antirheumatic drugs (DMARDS), drugs used for treatment of gout, commonly used nonsteroidal antnflammatory drugs (NSAIDS) and the newer COX-2 inhibitors.

Steady-state pharmacokinetics and tissue distribution of anthraquinones of Rhei Rhizoma in rats

AIM OF THE STUDY

Rhei Rhizoma, the rhizome of Rheum palmatum L. (RP), is a popular herb in clinical Chinese medicine. RP is abundant in polyphenolic anthraquinones, which have been reported to show various beneficial bioactivities. This study investigated the pharmacokinetics and tissue distribution of anthraquinones following seven-dose administration of RP decoction to rats.

MATERIALS AND METHODS

Six Sprague-Dawley rats were given 2.0 g/kg of RP twice daily for seven doses and blood samples were collected at designated time after the 7th dose. Another six rats were sacrificed at 30 min after the 7th dose and organs including liver, kidney, lung and brain were collected. Serum and tissue specimens were assayed by HPLC before and after hydrolysis with β-glucuronidase and sulfatase, respectively.

RESULTS

Pharmacokinetic analysis indicated that the anthraquinones in serum mainly presented as glucuronides/sulfates and contained higher ratio of sulfates when compared with single-dose administration of RP. Contrary to the finding in serum, tissue analysis discovered mainly free form of anthraquinone in most organs assayed, such as aloe-emodin and rhein in kidney, liver, lung; emodin in liver, lung; trace of chrysophanol in kidney and liver. In all brains, neither free forms nor their glucuronides/sulfates have been detected.

CONCLUSIONS

The glucuronides/sulfates of anthraquinones were the major forms in bloodstream, whereas the free forms of most anthraquinones were predominant in kidney and liver.

Human variability for metabolic pathways with limited data (CYP2A6, CYP2C9, CYP2E1, ADH, esterases, glycine and sulphate conjugation)

Human variability in the kinetics of a number of phase I (CYP2A6, CYP2C9, CYP2E1, alcohol dehydrogenase and hydrolysis) and phase II enzymes (glycine and sulphate conjugation) was analysed using probe substrates metabolised extensively (>60%) by these routes. Published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and available data on subgroups of the population (effects of ethnicity, age and disease) were abstracted using parameters relating primarily to chronic exposure [metabolic and total clearances, area under the plasma concentration time-curve (AUC)] and acute exposure (C(max)). Interindividual differences in kinetics for all these pathways were low in healthy adults ranging from 21 to 34%. Pathway-related uncertainty factors to cover the 95th, 97.5th and 99th centiles of healthy adults were derived for each metabolic route and were all below the 3.16 kinetic default uncertainty factor in healthy adults, with the possible exception of CYP2C9*3/*3 poor metabolisers (based on a very limited number of subjects). Previous analyses of other pathways have shown that neonates represent the most susceptible subgroup and this was true also for glycine conjugation for which an uncertainty factor of 29 would be required to cover 99% of this subgroup. Neonatal data were not available for any other pathway analysed.

Human variability in glucuronidation in relation to uncertainty factors for risk assessment

The appropriateness of the default uncertainty factor for human variability in kinetics has been investigated for glucuronidation using an extensive database of substrates metabolised primarily by this pathway. Inter-individual variability was quantified for 15 compounds from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve (AUC)) and acute exposure (C(max)). Low inter-individual variability (about 30-35%) was found for all parameters (clearance corrected or not corrected for body weight, metabolic clearance, oral AUC and C(max)) after either iv or oral administration to healthy adults. The overall variability of 31% for glucuronidation in healthy adults supported the validity of the default kinetic uncertainty factor of 3.16 for this group, because it would cover more than 99% of individuals. Comparisons between potentially sensitive subgroups and healthy adults using differences in means and variability indicated that neonates showed the greatest impairment of glucuronidation, and that the 3.16 kinetic default factor applied to the mean data for adults would be inadequate for this subpopulation. The in vivo data have been used to derive pathway-related default factors for compounds eliminated largely via glucuronidation.

Glucuronidation of diflunisal in liver and kidney microsomes of rat and man

1. The glucuronidation of diflunisal to its phenolic (DPG) and acyl glucuronide (DAG) was measured in vitro using microsomes prepared from rat (n = 4) and human (n = 6) liver and kidney tissue. UGT activities towards bilirubin, 4-nitrophenol and (-)-morphine were also determined. 2. beta-Glucuronidase activity towards phenolphthalein glucuronide was much lower in microsomes prepared from human liver (45.2 +/- 3.1 Fishman Units/mg protein), human kidney (22.0 +/- 3.3 FU/mg), and rat kidney (25.1 +/- 2.5 FU/mg) as compared with rat liver (118.7 +/- 8.8 FU/mg). 3. The formation rate of DAG significantly increased when saccharo-1,4-lactone, a beta-glucuronidase inhibitor, was added to the rat liver microsomal incubation medium. beta-Glucuronidase inhibition, however, had little effect on the formation rate of DAG in human liver microsomes, and no effect in rat and human kidney microsomes. The formation of DPG was not affected by the microsomal beta-glucuronidase activity. 4. Unlike rat kidney microsomes, which only formed DAG, human kidney microsomes formed both diflunisal glucuronides. Formation of both diflunisal glucuronides in human kidney microsomes (Vmax = 0.97 +/- 0.21 and 0.27 +/- 0.07 nmol/min/mg for formation of DAG and DPG respectively) represented 60-70% of the activity found in liver microsomes (Vmax = 1.58 +/- 0.32 and 0.40 +/- 0.08 nmol/min/mg for formation of DAG and DPG respectively). 5. These results demonstrate that the in vitro glucuronidation rate of diflunisal may be affected by the microsomal beta-glucuronidase activity particularly when using rat liver microsomes. Our results also demonstrate that the human kidney has an important UGT-activity towards diflunisal.

Concentration-dependent plasma protein binding of flurbiprofen in the rat: an in vivo microdialysis study

PURPOSE

The in vivo plasma protein binding and pharmacokinetics of flurbiprofen were studied in awake, unrestrained rats using intravenous microdialysis sampling.

METHODS

Flurbiprofen (20 mg/kg) was administered i.v. to 2 groups of 6 rats: in both groups sampling was carried out by microdialysis, but in the second group an additional 10 blood samples were withdrawn via a jugular cannula. In vitro and ex vivo (following i.v. administration of flurbiprofen 20 mg/kg to another group of 13 rats) plasma protein binding of the drug was determined by equilibrium dialysis.

RESULTS

The area under the unbound plasma concentration-time profile of flurbiprofen (AUCu), determined by microdialysis sampling was somewhat smaller (-19%, p = 0.666) in the rats undergoing simultaneous serial blood sampling (2.21 +/- 0.36 micrograms.h/ml) as compared to the rats undergoing microdialysis sampling only (2.73 +/- 0.60 micrograms.h/ml). Comparison of total and unbound concentrations of flurbiprofen showed an in vivo plasma binding varying between 99.5% at low and 98.0% at high total flurbiprofen plasma concentrations. Plasma binding of flurbiprofen determined in vitro over the same concentration range was higher (99.5-99.9%) but also concentration-dependent. Plasma binding of flurbiprofen determined ex vivo, on the other hand, corresponded well with the in vivo binding.

CONCLUSIONS

Monitoring the fraction of drug unbound in blood of an individual rat throughout a pharmacokinetic experiment has now become possible by using simultaneous sampling of blood and intravenous microdialysates.

Effect of probenecid on the formation and elimination kinetics of the sulphate and glucuronide conjugates of diflunisal

The effect of probenecid on the pharmacokinetics of diflunisal and its glucuronide and sulphate conjugates was studied in 8 healthy volunteers. Diflunisal 250 mg b.d. was administered p.o. for 15 days and its steady state pharmacokinetics was evaluated on Day 16 after the last dose (control phase). Probenecid 500 mg b.d. was co-administered throughout the entire study period in the treatment phase of the study. The steady state plasma concentration of diflunisal was significantly higher during the probenecid treatment phase as compared to the control phase (104.0 vs. 63.1 micrograms.ml-1). This was the result of a significant decrease in the plasma clearance of diflunisal from 5.8 (control) to 3.4 ml.min-1 (probenecid co-administration). The metabolite formation clearances of both glucuronides were significantly decreased by probenecid, -45% and -54% for the phenolic and acyl glucuronide, respectively. The metabolite formation clearance of the sulphate conjugate was not affected by probenecid coadministration. Steady state plasma concentrations of the sulphate and glucuronide conjugates of diflunisal were 2.5- to 3.1-fold higher during probenecid co-administration, due to a significant reduction in the renal clearance of the three diflunisal conjugates. Probenecid also reduced the plasma protein binding of diflunisal, but only to a minor extent; the unbound plasma fraction of diflunisal at steady state averaged between 5 and 30% higher during probenecid co-administration.

Studies on the reactivity of acyl glucuronides--VI. Modulation of reversible and covalent interaction of diflunisal acyl glucuronide and its isomers with human plasma protein in vitro

Acyl glucuronide conjugates are chemically reactive metabolites which can undergo hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions with protein. The present study was undertaken to identify factors modulating the reactivity of diflunisal acyl glucuronide (DAG) with human serum albumin (HSA) in vitro, by comprehensively evaluating the interplay of the three pathways above when DAG and a mixture of its 2-, 3- and 4-isomers (iso-DAG) were incubated with protein. Buffer, plasma, fraction V HSA, fatty acid-free HSA, globulin-free HSA and fatty acid- and globulin-free HSA were investigated at pH 7.4 and 37 degrees, each in the absence and presence of warfarin, diazepam and diflunisal (DF) as reversible binding competitors. DAG and iso-DAG were highly reversibly bound (ca. 98-99.5%) in plasma and HSA solutions. The binding was primarily at the benzodiazepine site, since displacement occurred in the presence of diazepam and fatty acids but not warfarin. DAG degradation, via rearrangement, hydrolysis and covalent adduct formation (in that order of quantitative importance), was retarded in plasma and HSA solutions compared to buffer. The protective effect of protein was afforded by the high reversible binding to the (non-catalytic) benzodiazepine site. The warfarin site appeared to be catalytic for DAG hydrolysis, whereas rearrangement appeared to be hydroxide ion-catalysed only. In contrast to DAG, iso-DAG degradation was greatly accelerated in the presence of protein, through both covalent binding and catalysis of hydrolysis. Covalent binding via DAG was increased in the presence of warfarin but decreased in the presence of diazepam, DF and fatty acids. The opposite effects were found for covalent binding via iso-DAG. The data suggest that covalent binding of DF to HSA via DAG and iso-DAG occurs by different mechanisms (presumably transacylation and glycation, respectively) at different sites (benzodiazepine and warfarin, respectively) whereas reversible binding occurs primarily at the same site (benzodiazepine).

Glucuronidation of diflunisal by rat liver microsomes. Effect of microsomal beta-glucuronidase activity

The in vitro formation rates of the phenolic (DPG) and acyl (DAG) glucuronides of diflunisal were investigated using rat liver microsomes. Preliminary studies showed that DAG hydrolysed rapidly (T1/2 = 12 min) when incubated in the presence of rat liver microsomes at pH 7.4 and 37 degrees. DPG was much more stable under the same conditions (T1/2 = 35 hr). Hydrolysis of DAG and DPG by rat liver microsomes was inhibited by 4 mM saccharolactone, a beta-glucuronidase inhibitor. The apparent Km and Vmax values for the formation of DAG in the absence and presence of 4 mM D-saccharic acid-1,4-lactone (saccharolactone) were the following: Km = 0.05 +/- 0.02 vs 0.08 +/- 0.02 mM and Vmax = 0.20 +/- 0.06 vs 0.43 +/- 0.07 nmol/min/mg protein (0 and 4 mM saccharolactone, respectively). The significant increase in apparent Vmax for DAG formation in the presence of saccharolactone can be explained by the inhibition of beta-glucuronidase-catalysed hydrolysis of DAG. Apparent Km and Vmax values for the formation rate of DPG were not affected by addition of saccharolactone to the incubation medium. These results indicate that beta-glucuronidase-catalysed hydrolysis of certain glucuronides formed during microsomal incubations may significantly affect the apparent glucuronidation rate due to the presence of a glucuronidation-deglucuronidation cycle.

Studies on the renal excretion of the acyl glucuronide, phenolic glucuronide and sulphate conjugates of diflunisal

1. In five healthy male volunteers given multiple doses of diflunisal (DF), renal clearances (CLR) of the acyl glucuronide (DAG), phenolic glucuronide (DPG) and sulphate (DS) conjugates were about 42, 25 and 13 ml min-1, respectively. 2. These relatively low CLR values are probably due largely to the very high plasma protein binding of the conjugates, found in vitro to be 99.0%, 97.8% and 99.45%, respectively. 3. Thus glomerular filtration plays the minor and active tubular secretion the major role in renal excretion of the three conjugates. 4. This conclusion was supported by the effect of probenecid co-administration, which decreased CLR of DAG and DPG by about 70%. CLR for DS could not be calculated when probenecid was co-administered (because of interference by probenecid metabolites in the analysis of DS in urine). 5. Water-induced diuresis had no effect on CLR of the DF conjugates, consistent with tubular reabsorption being negligible.

Both phenolic and acyl glucuronidation pathways of diflunisal are impaired in liver cirrhosis

The pharmacokinetics of diflunisal, a salicylate derivative that undergoes phenolic and acyl glucuronidation as well as sulphate conjugation, has been studied after a single oral dose (250 mg) in patients with cirrhosis (n = 5) and in healthy controls (n = 5). The plasma clearance of total (bound + unbound) diflunisal was 10.2 ml.min-1 in the control subjects and it was not affected by cirrhosis (10.9 ml.min-1). The plasma protein binding of diflunisal was significantly reduced in cirrhosis; the percentage of unbound diflunisal in plasma was 0.089 in the controls and 0.147 in the patients with cirrhosis. Plasma clearance of unbound diflunisal was significantly impaired in cirrhosis: 11.5 l.min-1 in control subjects vs 7.41.min-1 in cirrhotics. In cirrhotic patients, the unbound partial clearances to the phenolic and acyl glucuronides were both significantly reduced, by approximately 38%. The unbound partial clearance to the sulphate conjugate was not significantly affected by cirrhosis. The results show that both the phenolic and acyl glucuronidation pathways of diflunisal are equally susceptible to the effects of liver cirrhosis.

Studies on the reactivity of acyl glucuronides--II. Interaction of diflunisal acyl glucuronide and its isomers with human serum albumin in vitro

A major metabolite of diflunisal (DF) is its reactive acyl glucuronide conjugate (DAG) which can undergo hydrolysis (regeneration of DF), intramolecular rearrangement (isomerization via acyl migration) and intermolecular reactions with nucleophiles. We have compared the fate of DAG and its individual 2-, 3- and 4-O-acyl positional isomers (at ca. 55 micrograms DF equivalents/mL) after incubation with human serum albumin (HSA, 40 mg/mL) at pH 7.4 and 37 degrees. Initial half-lives (T1/2) for DAG and its 2-, 3- and 4-isomers were 53, 75, 61 and 26 min, respectively. DAG was more labile to hydrolysis than any of its isomers but the latter, in particular the 4-isomer, were much better substrates for formation of covalent DF-HSA adducts. After a 2-hr incubation, 2.4, 8.2, 13.7 and 36.6% of substrate DAG and its 2-, 3- and 4-isomers (respectively) were present as DF-HSA adducts. With long term incubation, the concentrations of adducts so generated in situ declined in a biphasic manner, with apparent terminal T1/2 values of ca. 28 days. DAG was much more labile to transacylation with methanol (i.e. formation of DF methyl ester) than an equimolar mixture of its isomers after incubation in a 1:1 methanol:pH 7.4 buffer solution at 37 degrees (T1/2 values of 5 and 70 min, respectively). The data do not support direct transacylation with nucleophilic groups on protein as the predominant mechanism of formation of covalent DF-HSA adducts in vitro.

Absence of phenolic glucuronidation and enhanced hydroxylation of diflunisal in the homozygous Gunn rat

1. The disposition of diflunisal (DF) was investigated in bile-exteriorized and intact homozygous Gunn rats given 10 and 50 mg/kg doses i.v. and in Wistar rats given 10 mg/kg doses i.v. 2. In Gunn rats, DF sulphate, DF acyl glucuronide, and a hitherto unidentified metabolite of DF, a conjugate of 3-hydroxy-DF, were identified as the major metabolites, accounting for approximately 37%, 16% and 11% respectively of 10 mg/kg doses and 35%, 24% and 15% respectively of 50 mg/kg doses in bile-exteriorized animals. There was no evidence for formation of DF phenolic glucuronide. 3. Total plasma clearance of DF and formation clearances of DF to DF sulphate and 3-hydroxy-DF were little affected by increase of dose from 10 to 50 mg DF/kg, whereas formation clearance of DF to DF acyl glucuronide was increased, but not significantly. 4. In Gunn rats with undisturbed bile flow into the gut, recoveries of DF sulphate and total 3-hydroxy-DF in urine increased to approximately 48% and 25% dose respectively at the expense of DF acyl glucuronide through enterohepatic recirculation. 5. In bile-exteriorized Wistar rats, DF phenolic glucuronide, DF acyl glucuronide, DF sulphate and 3-hydroxy-DF accounted for 16%, 27%, 14% and 2%, respectively, of 10 mg/kg doses. In intact Wistar rats, urinary recoveries of the metabolites were 15%, 13%, 23% and 5%, respectively. 6. Thus in comparison to Wistar rats, phenolic glucuronidation of DF was absent or negligible in homozygous Gunn rats, acyl glucuronidation was significantly decreased, sulphation was unchanged, and the 3-hydroxylation of DF was significantly enhanced.

Identification of a hydroxy metabolite of diflunisal in rat and human urine

1. A new metabolite of diflunisal, a hydroxy derivative, has been identified in rat and human urine following administration of diflunisal. 2. This hydroxy metabolite of diflunisal is excreted in urine of both species as a polar conjugate, most likely a sulphate. 3. Attempts to isolate the polar conjugate in pure form were unsuccessful due to its rapid hydrolysis in the presence of acid, and organic solvents such as diethyl ether. Its breakdown product, however, was more stable and was isolated and purified by semi-preparative h.p.l.c. Unequivocal identification as 3-hydroxy-diflunisal (i.e. hydroxylation in position 3 of the salicylic acid ring) was accomplished by means of FAB-mass spectrometry and n.m.r. spectroscopy. 4. The contribution of this oxidative metabolic pathway to the overall elimination scheme of diflunisal is more important in rat than in man. Gunn rats excrete more of the hydroxy diflunisal conjugate in urine (20-30% of a 50 mg/kg i.v. dose of diflunisal) than Wistar rats. In healthy humans, hydroxylation of diflunisal contributes only to a small extent to the overall biotransformation of diflunisal.

Diflunisal and its conjugates in patients with renal failure

Six patients with renal failure were given a single oral dose (250 mg) of diflunisal. In contrast to the acyl glucuronide, the phenolic glucuronide and sulphate conjugates showed the capacity to accumulate in plasma, suggesting that systemic instability of the acyl glucuronide contributes, via hydrolysis, to plasma concentrations of diflunisal itself. Although earlier studies in renal failure patients have almost certainly underestimated diflunisal clearance (by overestimation of plasma diflunisal concentrations through unrecognized acidic hydrolysis of diflunisal sulphate during analysis), the present results suggest that the reported decrease in clearance was not attributable only to this analytical artifact.